The current paths in electronic assemblies that include processors are continually being required to handle ever-increasing amounts of current in order to power the processors. Processors typically require more power in order to operate at higher frequencies and to simultaneously perform numerous logic and memory operations. As processor power densities continue to increase, so too does the structural and thermal challenge of adhering electronic packages that include dies to a substrate (e.g., a motherboard).
One example method of attaching a die to a substrate includes soldering the die to the substrate and then injecting an underfill between the die and the substrate. Capillary flow causes the underfill to seal the area between the die and the substrate that is not occupied by the soldered areas of connection.
One drawback with soldering the die to the substrate is that the various components contract at different rates during bonding. Since the die, solder and substrate contract at different rates, stress forms within the die, solder and substrate as the solder hardens to bond the die to the substrate. In addition, placing the underfill between the die and the substrate after solder bonding serves to lock in the stress within the various components. The stress that exists within such assemblies may cause cracks within the die, solder and/or substrate.
The stress within the various components makes the electronic assemblies vulnerable to unwanted cracking, especially when a motherboard is mounted within a chassis that is shipped to an end user. The shock and vibration forces that are generated during shipping can be particularly detrimental to such electronic assemblies.
One recent method of attaching a die to a substrate includes thermal compression bonding (TCB) the die to the substrate. A typical TCB process includes covering solder balls on a substrate with an underfill and then positioning solder balls on a chip against the solder balls on the substrate. Heat and a force are simultaneously applied to the solder balls over a period of time to cause simultaneous solder interconnect reflow and underfill cure. One of the advantages of TCB over a conventional capillary flow process is that the extra processing steps that are associated with a capillary flow process (e.g., flux application, flux residue cleaning and secondary thermal curing of the underfill) are eliminated.
Despite numerous processing advantages TCB presently suffers from a major drawback in that the interconnect yield rate of the soldered connections is very low when fillers are contained in the underfill material. A significant amount of filler is typically required in an underfill material in order improve the reliability of the connection between a die and a substrate. As an example, reliability tests show that at least 50 percent by weight of fillers is required in an underfill in order to improve solder joint reliability.
The interconnect yield rate is typically lower than desired because the fillers which are normally used in the underfill are made of a relatively hard material that tends to become entrapped between the die and substrate bumps (or pads). This entrapment of the fillers/underfill sometimes prevents the die bumps from making adequate contact with substrate bumps such that solder joints are unable to properly form.
There have been attempts to address the filler entrapment problem by forming the die and substrate bumps with rounded tips. However, the rounded tips cause other concerns in that the rounded die bumps tend to slip over the rounded solder bumps as a force is applied during the TCB bonding process. This slipping between the rounded die and substrate bumps can cause the die and the substrate to become misaligned. The die and the substrate can become so misaligned that the interconnect yield rate between the die bumps and the substrate bumps may be adversely effected.